Method of estimating Phosphoric Acid and its Compounds. 181 
for expressing the relation of the primary wave-length of any of 
the fixed lines to its index of refract on, becomes universal and 
quite simple. Denoting the wave-length by X, and the corre- 
sponding index of refraction by 4, we have in every case 
rn” 
ares nis 
En 
where the exponent z is constant for the medium and tempera- 
ture, as are also the quantities «, and a,, being the index of 
elasticity and the optical abstract corresponding to that expo- 
nent. These two are ascertainable from the observed indices, in 
the same manner as are ¢€ and a for the first power of the nor- 
mals, and they will be found specified for each medium in 
Table I. 
The indices of refraction for the various media, as calculated 
from this general formula, are given in Table VI., while the 
observed indices are specified in Table VII. The differences 
between the calculated and observed indices are exhibited in 
Table VIII. 
[To be continued. ] 
XXIII. On a Simple and Expeditious Method of estimating Phos- 
phoric Acid and its Compounds, which is particularly applicable 
to the Analysis of Phosphatic Manures and the Ashes of Plants. 
By Epmunp W. Davy, M.B., M.R.I_A. §c., Professor of 
Agriculture and Agricultural Chemistry to the Royal Dublin 
Society*. 
“stiri want of some simple and expeditious method of esti- 
- mating phosphoric acid and its compounds has long been 
felt ; for though several means have been devised for the deter- 
mination of this acid under different circumstances, they are so 
complicated and require so much time in their performance, that 
they are quite unsuited for many cases where expedition is 
particularly required. 
After much investigation I have succeeded in devising a 
method which is very quickly performed, easy of execution, and 
capable of affording extremely accurate results. It is founded 
on the fact that phosphoric acid possesses a great attraction for 
the peroxide of iron, so that when a persalt of that metal is 
added to a solution containing phosphoric acid, an insoluble 
combination of the peroxide of iron with that acid is produced, 
which under particular circumstances has the following com- 
* Communicated by the Author, being part of a paper read before the 
Royal Dublin Society, January 11, 1860. 
Phil. Mag. 8. 4. Vol. 19. No, 126, March 1860. O 
